In some chips it is necessary that the external supply voltage generated by an external voltage source must first be regulated in order then subsequently to be able to be used for the chip core. The requisite voltage regulators require a reference voltage, which is generally generated in the chip itself. Two noise paths can occur in this case. The first noise path relates to the path from the external voltage source to the reference voltage source and from the reference voltage source to the voltage supply for the chip core. The second noise path relates to the path from the external voltage source to the voltage supply for the chip core. Taking account of the noise paths is of importance in particular because the reference voltage regulator that generates the reference voltage generally has poorer noise suppression than the supply voltage regulator in the chip core. If the noise in the reference voltage source is too high, the latter may, under certain circumstances, even be destroyed.
The prior art discloses a circuit for generating a supply voltage such as is shown in FIG. 1. A voltage regulator 1, having no particular precautions for noise suppression, is connected, on the input side, to a voltage input IN, at which an external supply voltage EXTVDD is present. The voltage regulator 1 generates a reference supply voltage REFVDD at its output, which voltage is passed to a reference voltage source 2. The reference voltage source 2 generates therefrom a reference voltage VREF, which is subsequently fed to a low-noise voltage regulator 3 via the first input 3.1 thereof. The external supply voltage EXTVDD applied to the voltage input IN is present at the second input 3.2 of the low-noise voltage regulator 3. The low-noise voltage regulator 3 then generates a supply voltage VDD, which can be tapped off at the output 3.4 of the low-noise voltage regulator 3. If the low-noise voltage regulator 3 additionally requires a regulated voltage supply, the latter can be made available to it as reference supply voltage REFVDD via the input 3.3, which is indicated by the dotted line in FIG. 1.
An embodiment of a circuit for a voltage supply as shown in FIG. 1 has the disadvantage, however, that the noise of the reference voltage supply is suppressed only to a limited extent, which has the effect that the supply voltage VDD at the output of the circuit may be noisy. The circuit for supplying voltage for the chip core as shown in FIG. 1 therefore has only limited noise suppression.
A further embodiment of a circuit for generating a supply voltage is shown from the prior art, said embodiment being shown in FIG. 2. In the same way as in FIG. 1, the external supply voltage EXTVDD is applied to the input IN of the circuit. The circuit in FIG. 2 differs from the circuit shown in FIG. 1 by the fact that the noisy voltage regulator 1 used in FIG. 1 is replaced by a low-noise voltage regulator 6 and also a simple reference voltage regulator 4, having no particular noise suppression. In this case, the second low-noise voltage regulator 6 is connected to the voltage input IN via its input 6.2. A first reference voltage VREF1 is formed from the external supply voltage EXTVDD with the aid of the voltage regulator 4, and is present at the input 6.1 of the low-noise voltage regulator 6.
In the case of the embodiment shown in FIG. 2, the reference voltage REFVDD is generated by means of the low-noise second voltage regulator 6. This embodiment has the following disadvantages, however. The additional second low-noise voltage regulator 6 requires more space on the chip. Further disadvantages are that the embodiment shown in FIG. 2 consumes more current and the switch-on duration is greater than in the case of the embodiment shown in FIG. 1. If the low-noise voltage regulator itself needs a regulated supply voltage, a further voltage regulator is required, which additionally takes up chip area.